The present invention relates to an arrangement for recirculation of exhaust gases of a supercharged combustion engine, without making the performance of the inferior due to the recirculated gases.
The technique known as EGR (Exhaust Gas Recirculation) is a known way of leading part of the exhaust gases from a combustion process in a combustion engine back, via a return line, to an inlet line for supply of air to the combustion engine. A mixture of air and exhaust gases is thus supplied via the inlet line to the engine's cylinders in which the combustion takes place. The addition of exhaust gases to the air causes a lower combustion temperature resulting inter alia in a reduced content of nitrogen oxides NOx in the exhaust gases. This technique is applied in both Otto engines and diesel engines.
The amount of air which can be supplied to a supercharged combustion engine depends on the pressure of the air but also on the temperature of the air. Supplying the largest possible amount of air to the combustion engine therefore entails cooling the compressed air in a charge air cooler before it is led to the combustion engine. The compressed air is cooled in the charge air cooler by ambient air flowing through the charge air cooler. The compressed air can thus be cooled to a temperature which is only a few degrees above the temperature of the surroundings. In cases where EGR technology is used, the returned exhaust gases also need cooling. This is achieved by means of a so-called EGR cooler. An EGR cooler is usually connected to the combustion engine's cooling system so that the exhaust gases are cooled in the EGR cooler by the circulating coolant in the cooling system. The EGR cooler is thus subject to the limitation that the exhaust gases cannot be cooled to a lower temperature than the temperature of the coolant in the cooling system. The cooled exhaust gases are therefore usually at a higher temperature than the cooled compressed air when they are put into the inlet line to the combustion engine. The mixture of exhaust gases and air being led to the combustion engine will therefore be at a higher temperature than the compressed air led into a supercharged combustion engine which is not provided with recirculation of exhaust gases. The performance of a supercharged combustion engine equipped with EGR is therefore somewhat inferior to that of a supercharged combustion engine not equipped with EGR.